1. Field of the Invention
The present invention relates to a catalyst for the purification of exhaust gases and, more particularly, to a catalyst mounted to an exhaust system for purifying exhaust gases by adsorbing hazardous components thereof.
2. Description of Related Art
Exhaust gases from vehicles are purified by removing their hazardous and noxious components such as NOx, CO and so on by, for example, ternary catalysts which may be disposed in the exhaust system.
U.S. Pat. No. 4,888,320 discloses a catalyst composed of a catalyst carrier with two coats deposited thereon. Specifically, a first coat is deposited on a surface of the catalyst carrier and a second coat is further deposited on a surface of the first coat and, more specifically, the first coat is comprised of alumina and the second coat is comprised of alumina containing cerium oxide CeO.sub.2, platinum Pt and rhodium Rh. As the second coat is formed on the alumina layer as the first coat, the second coat containing a catalyst constituent such as platinum can be made uniform in coat thickness, preventing the catalyst component from sintering.
Japanese Patent Unexamined Publication (kokai) No. 113,487/1975 discloses a catalyst for the purification of exhaust gases in which a platinum group metal is deposited on the coat layer formed on the catalyst carrier comprised of active alumina by preparing a slurry solution containing at least one of chromium and tungsten and at least one of calcium, strontium, barium, silicon, tin and zirconium and coating the slurry solution on the surface of the carrier.
Japanese Patent Unexamined Publication (kokai) No. 71,536/1987 discloses a catalyst consisting of a first coat layer comprised of alumina containing platinum and rhodium and a second coat layer, which is formed on the first coat layer, containing cerium oxide and palladium.
Further, Japanese Patent Examined Publication (kokoku) No. 14,338/1987 discloses technology of forming a coat on the surface of a catalyst carrier by preparing a slurry from a mixture of an active alumina with at least one of cerium, zirconium, iron and nickel and at least one of platinum, palladium and rhodium, coating the slurry on the surface of the carrier to form a wash coat as well as drying and calcining the wash coat.
It is to be noted that the coat with cerium oxide contained therein can exhibit the effect of storing oxygen due to the cerium oxide. This effect of storing oxygen is such that, when the concentration of oxygen is high, namely, when the air/fuel ratio is on the lean side, the oxygen is adsorbed while, when the concentration of oxygen is low, namely, when the air/fuel ratio is on the rich side, the oxygen is discharged, thereby contributing to the catalytic reaction of the catalyst. Therefore, due to the catalytic effect of storing oxygen to be achieved by the cerium oxide contained in the catalyst, the catalyst has an extended region of the air/fuel ratio in which the catalyst can react with nitrogen oxides (NOx), hydrocarbons (HC) and so on. Hence, cerium oxide contributes to improvements in catalytic performance. It is also known that cerium oxide itself can react with water to cause a reaction with hydrogen gas, thereby serving as improvements in catalytic performance. Cerium oxide, however, may have the tendency that its crystal grows and its crystalline structure is rendered so large that a specific surface area of the cerium oxide becomes small. A smaller specific surface area of the cerium oxide results in a reduction in the catalytic effect of storing oxygen and in the reaction with hydrogen gas. Such a tendency that cerium oxide contained in the catalyst causes heat deterioration on account of the crystal growth due to a high-temperature heat applied to the catalyst from exhaust gases suffers from the difficulty to stabilize the catalytic activity of the catalyst with the cerium oxide contained therein at low temperatures.